Return Mode
Last updated
Last updated
The Return flight mode is used to fly a vehicle to safety on an unobstructed path to a safe destination, where it may either wait (hover or circle) or land.
PX4 provides several mechanisms for choosing a safe return path, destination and landing, including using home location, rally ("safe") points, mission paths, and mission landing sequences.
The following sections explain how to configure the , and . At the end there are sections explaining the default (preconfigured) behaviour for each .
:::note
This mode requires a global 3d position estimate (from GPS or inferred from a ).
This mode is automatic - no user intervention is required to control the vehicle.
RC control switches can be used to change flight modes on any vehicle.
RC stick movement in a multicopter (or VTOL in multicopter mode) will change the vehicle to unless handling a critical battery failsafe. :::
PX4 provides four alternative approaches () for finding an unobstructed path to a safe destination and/or landing:
(RTL_TYPE=0
): Ascend to safe altitude and return via a direct path to the closest rally point or home location.
(RTL_TYPE=1
): Ascend to a safe altitude, fly direct to the closest destination other than home: rally point or start of mission landing. If no mission landing or rally points are defined, return home via direct path.
(RTL_TYPE=2
): Use mission path and fast-continue to mission landing (if defined). If no mission landing defined, fast-reverse mission to home. If no mission defined, return direct to home (rally points are ignored).
(RTL_TYPE=3
): Ascend to a safe altitude and return via direct path to closest destination: home, start of mission landing pattern, or rally point. If the destination is a mission landing pattern, follow the pattern to land.
More detailed explanations for each of the types are provided in the following sections.
In this return type the vehicle:
Flies via direct path to the home position or a rally point (whichever is closest)
:::note If no rally points are defined, this is the same as a Return to Launch (RTL)/Return to Home (RTH). :::
In this return type the vehicle:
If the destination is a mission landing pattern it will follow the pattern to land.
:::warning When this type is set PX4 will reject any mission without a valid landing pattern. :::
This return type uses the mission (if defined) to provide a safe return path, and the mission landing pattern (if defined) to provide landing behaviour. If there is a mission but no mission landing pattern, the mission is flown in reverse. Rally points, if any, are ignored.
:::note The behaviour is fairly complex because it depends on the flight mode, and whether a mission and mission landing are defined. :::
Mission with landing pattern:
Mission mode: Mission is continued in "fast-forward mode" (jumps, delay and any other non-position commands ignored, loiter and other position waypoints converted to simple waypoints) and then lands.
Auto mode other than mission mode:
Fly directly to closest waypoint (for FW not a landing WP) and descend to waypoint altitude.
Continue mission in fast forward mode from that waypoint.
Manual modes:
Fly directly to landing sequence position and descend to waypoint altitude
Land using mission landing pattern
Mission without landing pattern defined:
Mission mode:
Mission flown "fast-backward" (in reverse) starting from the previous waypoint
Jumps, delay and any other non-position commands ignored, loiter and other position waypoints converted to simple waypoints.
VTOL vehicles transition to FW mode (if needed) before flying the mission in reverse.
Auto mode other than mission mode:
Fly directly to closest waypoint (for FW not a landing WP) and descend to waypoint altitude.
Continue the mission in reverse, exactly as though Return mode was triggered in mission mode (above)
Manual modes: Fly directly to home location and land.
If no mission is defined PX4 will fly directly to home location and land (rally points are ignored).
If the mission changes during return mode, then the behaviour is re-evaluated based on the new mission following the same rules as above (e.g. if the new mission has no landing sequence and you're in a mission, the mission is reversed).
In this return type the vehicle:
Flies a direct path to the closest destination of: home location, mission landing pattern or rally point.
If the destination is a mission landing pattern the vehicle will follow the pattern to land
A vehicle will usually first ascend to a safe altitude before returning, in order to avoid any obstacles between it and the destination.
If the vehicle is:
Inside the cone:
Note:
the vehicle returns at RTL_RETURN_ALT
(or above).
The mode is implemented in almost exactly the same way in all vehicle types (the exception being that fixed wing vehicles will circle rather than hover when waiting), and are hence tuned using the same parameters.
However the default configuration is tailored to suit the vehicle type, as described below.
Fly to the home position in a straight line and constant altitude (if already above the return altitude it will return at its current altitude).
If a mission landing is defined, fly direct to the mission landing start point and then land.
RC stick movement is ignored.
If a mission landing is defined, fly direct to the mission landing start point and then land.
Otherwise fly directly to the home position, transition to multicopter mode, and land as a multicopter.
Return mechanism (path and destination).
0
: Return to a rally point or home (whichever is closest) via direct path.
1
: Return to a rally point or the mission landing pattern start point (whichever is closest), via direct path. If neither mission landing or rally points are defined return home via a direct path. If the destination is a mission landing pattern, follow the pattern to land.
2
: Use mission path fast-forward to landing if a landing pattern is defined, otherwise fast-reverse to home. Ignore rally points. Fly direct to home if no mission plan is defined.
3
: Return via direct path to closest destination: home, start of mission landing pattern or safe point. If the destination is a mission landing pattern, follow the pattern to land.
Minimum return altitude and altitude at which the vehicle will slow or stop its initial descent from a higher return altitude (default: 30m)
Time to hover at RTL_DESCEND_ALT
before landing (default: 0.5s) -by default this period is short so that the vehicle will simply slow and then land immediately. If set to -1 the system will loiter at RTL_DESCEND_ALT
rather than landing. The delay is provided to allow you to configure time for landing gear to be deployed (triggered automatically).
Minimum horizontal distance from home position to trigger ascent to the return altitude specified by the "cone". If the vehicle is horizontally closer than this distance to home, it will return at its current altitude or RTL_DESCEND_ALT
(whichever is higher) instead of first ascending to RTL_RETURN_ALT).
Half-angle of the cone that defines the vehicle RTL return altitude. Values (in degrees): 0, 25, 45, 65, 80, 90. Note that 0 is "no cone" (always return at RTL_RETURN_ALT
or higher), while 90 indicates that the vehicle must return at the current altitude or RTL_DESCEND_ALT
(whichever is higher).
Ascends to a safe (above any expected obstacles).
at descent altitude (depending on landing parameters).
Ascends to a safe (above any expected obstacles).
Flies via direct path to a rally point or the start of a (whichever is closest). If no mission landing or rally points are defined the vehicle instead returns home via a direct path.
If the destination is a rally point or home it will at descent altitude (depending on landing parameters).
:::note A mission landing pattern consists of a , one or more position waypoints, and a . :::
Ascend to a safe above any expected obstacles.
Ascend to a safe above any expected obstacles.
On reaching waypoint 1, the vehicle ascends to the and flies to the home position (where it ).
Ascends to a safe (above any expected obstacles).
If the destination is a home location or rally point, the vehicle will descend to the descent altitude () and then .
:::note This is true for most . The exception is when executing a from within a mission, where the vehicle follows mission waypoints (we can assume these avoid any obstacles). :::
The return altitude for a fixed-wing vehicle is configured using the parameter . The return altitude for multicopter and VTOL vehicles is configured using the parameters and , which define a half cone centered around the destination (home location or safety point).
Above (1) it will return at its current altitude.
Below the cone it will return where it intersects the cone (2) or (whichever is higher).
Outside the cone (3) it will first climb until it reaches .
Above (4) it will return at its current altitude.
Below (5) it will first ascend to RTL_DESCEND_ALT
.
If is 0 degrees there is no "cone":
If is 90 degrees the vehicle will return at the greater of RTL_DESCEND_ALT
and the current altitude.
The vehicle will always ascend at least for the return.
Unless executing a mission landing (e.g. if executing a or ), the vehicle will arrive at its destination, and rapidly descend to the altitude.
The vehicle will the loiter for a specified time () and then land. If it will loiter indefinitely.
Multicopters use a by default (and the following configuration):
Ascend to ( - cone not used).
Rapidly descend to the altitude.
Land more or less immediately (small ).
Fixed-wing aircraft use a by default:
Otherwise fly directly to the home position and circle above it at radius .
If not following a mission landing, and is set to -1, the vehicle will land as described in the topic: .
The fixed wing depends only on (the cone defined by is not used)
VTOL aircraft use a by default:
:::note If not in a mission landing, a VTOL in FW mode will always transition back to MC just before landing (ignoring ). :::
The RTL parameters are listed in (and summarised below).
Return altitude in meters (default: 60m) when is 0. If already above this value the vehicle will return at its current altitude.
Controls whether stick movement on a multicopter (or VTOL in MC mode) causes a mode change to (except when vehicle is handling a critical battery failsafe). This can be separately enabled for auto modes and for offboard mode, and is enabled in auto modes by default.
The amount of stick movement that causes a transition to (if is enabled).
[Fixed Wing Only] The radius of the loiter circle (at .